2002 chevy suburban 1500 lt transmission does not shift past second gear.

Probably clutch plates but since they are slipping that means the particles could have easily gotten into the rest of the trans and may not cause problems right away or even at all but at the same time it may require you to rebuild in 6 months or a year anyway. It seems odd that a shop would only replace the fricton plates and not everything else. Were they both trans rebuild shops? In a case where more than $500 is asked for repairs most people would recommend 2 out of 3. If you take it two places and both say the exact thing then pick the cheaper of the two. If the two places have different stories then take it to a third and see if it matches either of the previous ones. Remember to always play dumb, act as though their shop is the first you are visiting. Good example is you get an estimate and it will cost $700, you go to the next place and they find $100 in repairs needed. You have already mentioned what the other place said and the second shop says that's crazy I can have this running great for $400. You see that as an awesome deal and don't even realise you paid 4X what you should have.